Tracking Birds at Feeders Using Existing Technology

It’s amazing what you can learn about birds when they can be identified as individuals, rather than being just another beak in the crowd. For years, scientists have kept track of who’s who by attaching colored bands to birds’ legs and then keeping a vigilant eye out for the marked bird to return. Now, a technology called RFID (radio frequency identification) is automating that vigilance, and yielding more detailed information than scientists ever dreamed possible. A recent study at the Cornell Lab kept track of 129 separate songbirds on a staggering 650,000 individual feeder visits over a period of 5 months (see “What We’ve Learned Already,” below). Instead of pulling 24-hour vigils, the scientists needed to spend only about 6 hours per week tending these wired feeders.

RFID tags have all kinds of uses—many people tag their pets in case they get lost, new passports use them to store your identity and even your picture, and shipping companies use them to keep track of where merchandise is on its way to you. All it takes is a tiny, battery-free tag and a larger tag reader and antenna. The reader bounces a signal off the tag and gets a unique ID number back in response. Biologists began using RFID in the 1990s to monitor hordes of birds without having their field assistants go cross-eyed looking for bands. A German team tracked the movements of more than 1,000 Common Terns over an 11-year period on two islands. Another group uses the devices to automatically weigh penguins as they feed their chicks on Antarctic beaches. RFID’s main drawback has always been price. At $800 to $10,000 per tag reader, ornithologists can’t really buy them by the dozen. But David Bonter, Project FeederWatch director at the Cornell Lab, and Eli Bridge of the University of Oklahoma have developed a do-it-yourself version that can be put together in a garage for a total cost of about $40. They published their technique in a forthcoming issue of the Journal of Field Ornithology, and posted instructions online (www.bit.ly/diyRFID, see also “How RFID Works”, below).

In a five-month pilot study this past winter, Bonter, research associate Ben Zuckerberg, and a team of Cornell University students put tags on 129 Black-capped Chickadees, White-breasted Nuthatches, Tufted Titmice, and House Finches. When the birds visited feeders equipped with tag readers placed in nearby woods, the data started pouring in—8,000 hours of continuous observations at a cost of just 6 hours of feeder upkeep per week. Some of the results were surprising:

Individual birds took up to 203 seeds in a single day (some of these they almost certainly cached for later in the season).

Most chickadees had favorite feeders—one or two locations they habitually visited even though several other feeders were available within the typical home-range area of a chickadee.

Over three months, a single Tufted Titmouse drifted through the woods, frequenting three feeders one after another across a distance of more than half a mile.

A Black-capped Chickadee spent two months visiting a feeder daily, then abruptly moved nearly a half-mile away to a different feeder.

The feeder/readers were very accurate—garbled identifications happened less than one percent of the time. By comparison, humans recording color bands make mistakes five percent of the time or more.

We can answer all sorts of questions about bird feeding and breeding behavior using this technique, such as: When do birds feed during the day? How is feeding behavior affected by weather? What influence does feeder location have on feeding behavior?

Which Species?

Bonter’s and Zuckerberg’s study at the Cornell Lab now includes Black-capped Chickadees, Tufted Titmice, White-breasted Nuthatches, Downy Woodpeckers, House Finches, and American Goldfinches. Cornell graduate student Jon DeCoste uses the feeders to study disease in House Finches. Other researchers have used RFID to study (not always using feeders) Common Terns, Semipalmated Sandpipers, Leach’s Storm-Petrels, pheasants, bobwhites, domestic chickens and ducks, three species of penguins, and endangered species such as the Florida Scrub-Jay and the Kakapo of New Zealand. Numerous studies have checked to see if the tags cause any harm to the birds who carry them, and have found no ill effects.

How RFID Works

It’s always been difficult to follow small birds with technology—the devices weigh too much. With RFID, the heavy battery sits on a frequently visited structure such as a feeder, freeing the bird to wear a very light transponder.

1. A tiny tag, weighing less than 0.1 gram, is securely taped to a band on the bird’s leg.

2. An antenna lining the feeder perch “looks” for a tag every second. A battery or solar panel powers the reader.

3. The bird’s tag contains no battery. When it enters the antenna’s range (about 5 inches), it returns an ID number. A mini-computer records it along with the time.

4. Even visits too brief for a human to correctly note are captured.

5. Because the system never sleeps, it can generate a complete record of the complex lives and interactions of feeder birds.